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An enormous airship built for the US forces has been bought back by its British designers and is to go into commercial service based in old Blighty.

Regular Reg readers will already be familiar with the ship, formerly designated as the first of the US Army's* planned fleet of Long Endurance Multi-intelligence Vehicle (LEMV) ships. It was expected that LEMVs, operating without crews aboard, would lurk for weeks on end high in the skies above Afghanistan, carrying spy podules to provide an unwinking watch over huge swathes of territory.

The current winding down of the Wars On Stuff, coupled with sweeping US government budget cuts following the recent economic troubles, saw the LEMV programme cancelled not long after the first ship flew at Lakehurst in New Jersey back in 2012 (famously the place where the mighty zeppelin Hindenburg burned and crashed in 1937, putting an end to the era of the great pre-war rigid ships).

The LEMV had actually been designed by a British firm, Hybrid Air Vehicles, latest in a long line of companies run by UK airship advocates. After the US Army cancellation, HAV negotiated successfully to buy the vessel. It was then taken apart and transported to Britain, and is now being restored to airworthy condition in the famous Number One shed at Cardington - the colossal building in which Blighty's answer to the Hindenburg, the likewise disastrous R101, was built.

The former LEMV - now to be known as the "Airlander" - was built to the HAV 304 design: the company has pulled the details of this off its website for some reason, but we had them noted. The ship is 91m long, 34m wide and 26m high - colossal, but even so it looks small inside the brobdingnagian Cardington hangar. The massive envelope maintains its shape by internal gas pressure, blimp-style, and is intended to generate extra dynamic lift over and above that from its helium filling as the ship flies along. The vessel will normally be heavier than air overall - it won't lift off the ground on its own. This is what is meant by a "hybrid airship".

The Airlander's engines are four 350hp, 4 litre supercharged V8 diesels, and the thrust they generate can be vectored upwards to get her off the ground. Once underway with forward speed generating dynamic lift, the bow engines are shut down in the cruise.

The LEMV was intended to operate mainly unmanned, but a single-pilot cockpit was provided for such occasions as transiting through normal civil airspace where a pilot is required for legal reasons. Apart from the cockpit, the Army version had a payload compartment for surveillance electronics and a "universal load beam". As a spy ship, the LEMV would have had a helium fill allowing operations up to 20,000 feet, and fuel to stay up for 21 days. It could carry 2,500lb (1134kg) of payload on such missions.

Pressure at 20,000 feet being about 0.45 of what it is at sea level, one would expect about 45 per cent of a LEMV's envelope volume to be helium at sea level, with the rest occupied by air in internal ballonets: as the ship climbed the air would gradually be expelled by the expanding helium until the "pressure height" ceiling was reached at 20,000 feet.

Total envelope volume of a HAV 304 is 38,000 cubic metres, so total gas lift of a LEMV at sea level would be about 18.8 tonnes. Given that we're told this might be supplemented by as much as two-thirds again by dynamic lift, maximum all-up weight as a LEMV would be in the region of 30 tonnes.

As the Airlander, we are told that the ship will carry a crew and "a host of celebrities" - plus a couple of competition winners - on its maiden British flight in 2016, so it would seem that the ship will be substantially reconfigured. As we discussed a while ago, it wouldn't be hard to put more helium in it than it would have carried as a LEMV: this would lower its ceiling, but allow the ship to lift more off the ground. Likewise, the Airlander will probably carry less fuel as it will no longer need 21 days of endurance, and this too will allow more payload.